Probiotics for Depression in Cancer Patients

Anxiety and depressive disorders are quite prevalent in the general population. In 2015, it was estimated that 3.6% of the world population (around 264 million individuals) live with depression and 4.4% (around 322 million) with anxiety, while in the recent years of the COVID-19 pandemic, the prevalence of these disorders has significantly increased. According to the American Psychiatric Association, in 2013 depression was defined as a serious mental illness characterized by decreased mood lasting more than 15 days, a lack of interest, and (even) apathy to normal daily activities. Depression, particularly in its severe form - the major depressive disorder (MDD), is the second leading cause of disability and the most common emotional disorder. Recent research data indicate that this psychiatric disorder, as also similar neuropsychological conditions (Parkinson's, autism, etc.) have, as their main substrate, a disturbance in the diversity of the gut microbiome. More specifically, it has been shown that there is a direct and bidirectional communication between gut and brain - the "gut-brain-axis". The gut, through its microbiome, communicates with the brain by means of neural, immunological, and metabolic pathways, either directly, through the vagus nerve or indirectly, either through the microbiome immunomodulation or the tryptophan pathway, or by the production of various neuroactive molecules. The disturbance in the diversity of the gut microbiome, termed dysbiosis, involves changes in the composition and the number of bacteria genera; and is causatively related to depression via the gut-brain axis. The transmission of nerve impulses, both via the hypothalamic-pituitary-adrenal axis and via the vagus nerve is disrupted by dysbiosis, leading to anxiety and depression, or, conversely, stressful conditions, more particularly emotional stress, which triggers an imbalance in the hypothalamic-pituitary-adrenal axis, leading to systemic immune responses and intestinal dysbiosis. This imbalance can clinically manifest with alterations in digestive tract function, knowledge accepted since the beginning of the previous century. Nowadays, there is considerable evidence that the gut microbiome represents a novel anti-depressant and the term "psychobiotics" has been used to describe those species of probiotic bacteria excreting mental health benefits. Certain Bifidobacterium and Lactobacillus species have already demonstrated, both experimentally and in clinical studies, their capability to improve mood, reduce anxiety, and enhance cognitive function. On these grounds, it is reasonable to raise the hypothesis that, by restoring the diversity of the gut microbiome, and thus enhancing the population of these specific probiotic species - termed 'psychobiotics', due to their specified functions, - could at least lead to an improvement in neuro-psychological disorders through gut-brain-axis remodeling.